translation - Meaning of a sentence with Aが言うのもなんだけど and other considerations

Basically my question is what does the following line mean:

A: あたしらが言うのもなんだけど……テストの相手があたしらで、テストになんの？

Context: Persons A and B are testing the combat abilities of one of two groups of people. To them unexpectedly person C arrives, with whom A and B have had mutual bad history (both sides did bad things). B reacts with surprise at the presence of C, but C behaves professionally and explains that she is there under orders and asks if there is a problem. Then the above line happens.

Now on to the line itself. I researched a bit, and there seems to be a possibility that が言うのもなんだけど is some sort of phrase. Allthough also after some time thinking about it, I can see it meaning:

That (is there a problem) is what we too meant/wanted to say/ask however,...

Which would make sense since they would want to know if the person means to start trouble. Which ties nicely into later B's comment to C about her not taking this chance to do revenge.

Then comes the later bit, which I assume has quite a bit of info that is supposed to be derived from context missing.

After some time looking at it, I think テストの相手があたしらで、might mean

because/since we are opponents in this test...

but I can't really tell what テストになんの？ means? I guess it might mean something along the line of A asking is this the test for her or something. But that is all contextual conjecture. In particular the になんの confuses me. I think it might be "に何の" but eh, I'm not 100% how that works then.

Answer

A:「あたしらが言{い}うのもなんだけど……テストの相手{あいて}があたしらで、テストになんの？」

I would like to start with a brief explanation of the two 「なん's」 because those are both important here.

The first 「なん」 is for 「何」, which you guessed correctly. I shall come back to this later.

The second 「なん」, however, is the colloquial form of the verb 「なる」. (Remember what I said about 「ん」 in colloquial speech in my answer to your last question?)

「テストになる」 means 「（良{よ}い）テストとして成立{せいりつ}する」 = "to be (or to serve as) a valid test"

You surely see 「成{な}る」 in the word 「成立」, yes?

Now back to the first 「なん」.. We obviously need to talk about what 「なんだけど」 means in this context as it would mean close to nothing if you literally translated it. 「なん」 here is a filler word for a more concrete word that has a negative meaning. Here, that would be a word like "strange", "weird", etc. In Japanese, 「変{へん}」 is often used when an actual word is used instead of a filler.

You will keep encountering the filler words such as 「なん/なに」, 「あれ」, etc. instead of "actual" negative words for as long as you study Japanese. Why so? Because that is simply how Japanese is spoken.

Putting everything together, we will have something like:

A: "It might sound kinda weird coming from us, but would that be a valid test if it were us used as (virtual) opponents in it?"

With more context, I could possibly give you a better TL. Working from just one sentence, however, I had to put the "virtual" in parentheses just to be safe.